1. Technical Field
This invention relates to devices for sensing low level analog signals, and particularly to a Josephson junction superconducting quantum interference device (SQUID) analog to digital converter.
2. Discussion
The problem of sensing low level analog signals and their conversion to a digital format for subsequent digital processing is important in a number of fields including magnetic imaging, nondestructive testing, magnetometers, gradiometers, infrared detectors and focal plane arrays, millimeter-wave detectors and arrays, and low-level instrumentation. In these applications, the analog signal is a low level current or magnetic field, and performance parameters such as power dissipation, linearity, noise, dynamic range, bandwidth and slew rate are important.
Extremely sensitive magnetometers have been developed using superconducting quantum interference devices (SQUIDS) which consist of two Josephson tunnel junctions connected in parallel on a superconduction loop. SQUID-counter based analog to digital converters (ADCs) generally rely on the application of fundamental properties of Josephson junction SQUIDS and techniques for manipulation of single (magnetic) flux quanta (SFQ) and superconducting integrated circuits such as SFQ counters and digital logic. SQUID counter-based ADCs usually have a least-significant-bit (lsb) which is the current required to produce a change of one flux quantum (.PHI..sub.0 =2.07 .times.10.sup.-15 WEBERS) in the SQUID: EQU I.sub.lsb =.PHI..sub.0 /M
where M is the mutual inductance between the input transformer (T) and the SQUID inductance. In prior ADC systems, the quantizing SQUIDs are operated in the zero voltage state, and the signal current produces SFQ changes which are detected either (1) by counting the number of transitions of a single SQUID with a SQUID flip-flop, or (2) by strobing an array of binary-coupled SQUIDs above a fixed gate current and decoding the voltage pattern. However, in general, prior SQUID ADCs have poor current sensitivity. Power dissipation, dynamic range, linearity and slew rate are also important considerations in the performance of SQUID ADCs.
Conventional analog SQUID sensors, used as magnetometers, gradiometers, or general galvanometers, are commonly implemented in a "flux-locked" mode which linearizes the basic periodic response of the SQUID by using analog signal processing and feedback. However, this technique frequently reduces the basic sensitivity and bandwidth.
Thus, it would be desirable to provide a SQUID ADC which provides significant improvements in sensitivity and dynamic range. Also, it would be desirable to have such a SQUID ADC which operates at high speed with minimum power dissipation.